eHealth Infrastructure (v2019.5.3) 

{
  "resourceType" : "Task",
  "id" : "ehealth-task-genexample",
  "groupIdentifier" : {
    "system" : "http://www.acme.com/identifiers/patient or urn:ietf:rfc:3986 if the Identifier.value itself is a full uri",
    "value" : "123456"
  }
}